Coating on aluminum



Patented Mar. 7, 1933 UNITED STATES PATENT OFFICE MARTIN TOSTERUD, OF ARNOLD, PENNSYLVANIA, ASSIGNOR, BY MESNE ASSIGNMENTS,

TO ALUMINUM COLORS INCORPORATED, OF INDIANAPOLIS, INDIANA, A CORPORA- TION OF DELAWARE COATING 0N ALUMINUM N0 Drawing.

This invention relates to the casting of aluminum or aluminum base alloys, herein and in the appended claims collectively defined by the term aluminum; The invention has particular reference to the formation on aluminum surfaces of hard, compact oxide coatings of uniform color and thickness, of good resistance to abrasion and of uniform hardness throughout their entire thickness.

Among the known methods of producing on aluminum surfaces oxide coatings, that method which comprises the use of sulphuric acid is particularly efficient and useful; the coatings produced thereby having many distinct and useful properties. According to general practice the aluminum to be coated is made the anode in an electrolytic cell the electrolyte of which is a strong solution of sulphuric acid. The oxide coat ngs thus produced are thick, are of good color and have many peculiar properties. Such coatings have, however, one distinct drawback, the exterior portions of the coatings ,are softer than the interior portions which lie adjacent to-the metal. The coating is thus of uneven hardness and where a hard, dense and perfectly compact coating is required which has a very high degree of resistance to abrasion the oxide coatings of uneven hardness and relatively soft exteriors have distinct disadvantages.

It is an object of this invention to provide an aluminum article having, on its surface, an oxide coating of emery-like hardness, consisting mainly of alumina, pervious in a sense to be hereafter explained, of substantial thickness but, withal, compact in structure,

uniform in color, smooth and enamel-like to.

the touch, and of uniform hardness throughout. Such useful coatings have not heretofore been provided on aluminum surfaces, the previously known coatings of similar nature being of variable hardness throughout their thickness with hard inner portions and relatively soft inner portions.

Such coatings are to be distinguished from the so-called oxide coating produced in film-like form on aluminum surfaces which have been exposed to the air. These latter Serial No. 561,991.

coatings are very thin and offer no substantial protection to the underlying metal while the oxide coatings of the present invention are of appreciable thickness, usually from about 0.0002 to 0.001 inch. Mqreover, the oxide coating produced on alunnnum which has been exposed to the air is impervious in the sense that the coating, once'formed, prevents further access of the coat-forming agents to the underlying aluminum surface, whereas the oxide coatings of the present invention are pervious in the sense that the coatforming agent may pass through the initially formed coating layer to act upon the underlying aluminum to build other and underlying layers of coating, thus producing a coating of greater thickness.

One method of producing such coatings comprises, as-I have discovered, making the aluminum to be coated the anode in an electrolyte consisting of a very dilute solution of sulphuric acid, that is to say, a solution containing not more than about 12 per cent of sulphuric acid by weight. I have discovered that when aluminum is made an anode in such an electrolyte, an oxide coating is produced which is not only of emery-like hardness but is also of-uniform hardness throughout and possesses a very high degree of resistance to abrasion. Inother respects the oxide coating thus formed in the aluminum is also eminently satisfactory. It is of good uniform grey color of pleasing appearance. It is, surprisingly, of a thickness comparable with or even thicker than coatings produced by concentrated sulphuric acid solutions. It is pervious. compact and smooth and enamel-like to the touch. It consists mainly of alumina.

The dilute solutions of sulphuric acid which may be used in producing oxide coatings of the novel properties above mentioned should preferably contain not more than about 12 per cent of sulphuric acid by weight and not less than about 5 per cent. It is preferable, from the standpoint of commercial production, to use solutions containing more than about 5 per cent sulphuric acid by weight since in such solutions a thick coating of the named characteristics is quickly produced when comparatively low voltages are impressed upon the aluminum anode. Solutions containing from about 5 per cent to about 12 per cent by weight of sulphuric acid are critical for the production of coatings of the nature described and are generally satisfactory.

The aluminum to be coated is made the anode in such a solution; a piece of aluminum, lead or carbon or the like will serve as a cathode. Preferably the aluminum should be thoroughly cleaned before the coating operation to avoid contamination of the electrolyte and to insure uniform coating action but cleansing is not a necessary element 0 the process and depends to a great extent upon the particular type of foreign material which may be on the metal surface. The temperature of the electrolyte during the anodic oxidation of the aluminum should, for best results, be maintained below about 35 degrees centigrade and, in practice, I prefer to maintain the temperature at about 20 to 25 degrees centigrade. The voltage which may be impressed upon the aluminum anode will vary somewhat with the strength of acid used in the electrolyte and the temperature. Usually a voltage of 20 to 30 will be found to be satisfactory. The time of treatment depends partially upon the voltage, the strength of acid in the electrolyte, and the thickness of coating desired. Using an electrolyte containing 7 per cent by weight of sulphuric acid at a temperature of 25 degrees centigrade and impressing 20 volts upon the anode, a treatment time of about 30 minutes has been found to be very satisfactory. Longtime treatment should, whenever possible, be avoided, the proper criterion being a treatment as brief as is compatible with desired results.

Although, by the skilled operator, it is readily determined that oxide coatings produced on aluminum in accordance with the principles of my invention are distinguished from coatings produced in electrolytes of higher sulphuric acid concentrations by a greater and a uniform hardness, the reasons for this important difference are not readily ascertainable. It is my belief, although such belief forms no part of my present invention, that the surprising results obtained by the use of dilute solutions of sulphuric acid are due to the absence, in such solutions, of any substantial propensity to dissolve appreciable amounts of the coating which is formed.

NVhereas in the case of electrolytes of high sulphuric acid concentrations it is my belief that the electrolyte attacks, during the coating process, the oxide coating first formed and tends to dissolve, and, consequently, to soften it.

The uniformly hard, pervious and compact oxide coatings produced on aluminum surfaces by the practice of my invention are particularly useful where an oxide coating to about 12 per cent by weight of sulphuric acid.

2. A process of producing on aluminum surfaces a thlck, compact and pervious oxide coating of emery-like and uniform hardnessand of uniform color and thickness, comprising making the aluminum to be coated an anode in an electrolyte containing about 5 to about 12 per cent by weight of sulphuric acld, and, during the formation of said coating, maintaining the electrolyte at temperatures of about 20 to about 35 centigrade.

3. As a new material, aluminum having ath 1ck, pervious, compact oxide coating cons 1st1ng mainly of A1 0 and having an emeryl1ke and uniform hardness throughout; said coating having the properties of one produced by the anodic treatment of aluminum in a 5 to 12 per cent by weight solution of sulphuric acid. 4. As a material, aluminum having a thick, pervious, compact oxide coating consisting mainly of A1 0 and having an emery-like and uniform hardness throughout; said coating having the properties of one produced by the anodic treatment of aluminum in a 5 to .12 per cent by weight solution of sulphuric 

